Heat-resistant alloy



Patented Mar. 26, 1935 Samuel L. Hoyt, Shorewood,

and Robert S.

Archer, Whitefish Bay, Wis., assignors to A. 0. Smith Corporation, Milwaukee, Wis., a corporation of New York No Drawing. Application March 12,1934,

Serial No. '715,158

'7 Claims.

This invention relates to an alloy which is resistant to oxidation at elevated temperatures.

An object of the invention is to provide an alloy of such properties as to be suitable for the fabrication of cast or wrought articles which are exposed to oxidizing conditions at elevated temperatures, such as annealing or carburizing boxes, or parts entering into the construction of conveyors, heat exchangers, recuperators, or other mechanism exposed to the action of oxidizing atmospheres at elevated temperatures.

Another object of the invention is to provide an alloy suitable for the construction of electrical resistance elements capable of operating at high temperature with a long life.

In accordance with this invention, an alloy of iron, chromium, and aluminum is provided which contains in excess of 35 per cent chromium and per cent aluminum, the balance being substantially all iron. The iron content of the alloy should be greater than the chromium content, and in the preferred composition, the iron content is greater than the combined chromium and aluminum contents. In general, as the chromium content is increased, the aluminum content should be also increased.

The composition which appears :to give the best results for the fabrication of resistance elements contains approximately 37% chromium, and 7% aluminum, the balance being substantially all iron. Alloys containing approximately these proportions of iron, chromium, and aluminum are highly useful in that their melting points are higher than the melting points of pure iron, or of binary alloys of iron and chromium containing 37% chromium, or of binary alloys of iron and aluminum containing 7% of aluminum.

When the melting points of ternary alloys of iron, chromium, and aluminum are plotted on a triangular diagram showing composition, it appears that alloys, having a composition near the composition corresponding to 37% chromium and 7% aluminum, are characterized by high melting points and that there is a range of composition in this vicinity in which the alloys have a melting point which is higher than 2700 F. and greater than the melting points of alloys having compositions represented by the points in the surrounding area of the triangular composition diagram. The alloy having 37% chromium and 7% aluminum has a melting point in excess of 2825" F. which is considerably higher than the melting point of iron. Due to their high melting point and great resistance to oxidation, the alloys in this region are exceptionally well adapted to use in high-temperature service.

The other elements which are normally present in steels may be present in the alloy in small amounts. The sulphur and phosphorus are desirably kept low as in ordinary steel. Titanium, when present to the extent of 0.50%, acts as a grain refiner and improves the physical properties of the cast alloy, but is not needed to increase resistance to scaling.

While the alloys can be prepared according to any of the usual methods suitable for this purpose, it has been found convenient to use a high frequency induction furnace. The iron and ferro-chromium or chromium are first melted together in a magnesia crucible in a high-frequency induction furnace and are protected from oxidation by a slag. A slag composed of lime and fiuorspar as a thinner has been found suitable for the purpose. Before the addition of the aluminum, the molten bath of iron and chromium may be deoxidized by the addition of calcium silicide.

When this is done, the completed alloy may contain up to half a per cent or even more silicon remaining from the deoxidizer. This deoxidation before the addition of aluminum is not indispensable, however, since excellent results have been obtained both from melts which have been deoxidized before the addition of aluminum,.and from melts which have not. In alloys which have not been deoxidized by calcium silicide. the silicon content is very much less than in alloys which have. V

The alloy may be cast into the articles which are desired if these are to be made by casting, or into ingots or billets to be subsequently worked into shape. These may be hot forged or rolled, or swaged into rods for electrical resistance elements. Ribbon for electrical resistance or heating elements can be formed by rolling.

The following specific example gives an illustration of a. particular alloy suitable for a heating unit and the results obtained on test. The alloy had the following composition:

Per cent Cr 37.38 'Al 7.03

N2 0.068 S and P Below 0.015

It was swaged into a Wire one eighth of an inch in diameter and tested for life. The life test was conducted by heating the wire by the passage of current for a period of three and three-quarters minutes. The current was then interrupted for the same period of time to allow the wire to cool to approximately room temperature, and the cycle was repeated. In this test, the current was automatically adjusted by means of a photo-electric cell to maintain. constant the temperature to which the wire is heated while the current is on. This method of procedure was found to be decidedly superior to the one in which a constant voltage is applied to the wire throughout the duration of the test. When a constant voltage is used, the temperature to which the wire is heated decreases as the test proceeds, and the-test conducted in this way not only gives no reliable information as to the life of a wire when heated to a. given temperature, but also furnishes less reproducible results than are obtained by the method used in these tests in which the wire is heated to a temperature that is maintained constant throughout the duration of the test.

. A one eighth inch wire of an alloy having the above-described composition was tested by heating to 2300 F., black body temperature as measured by an optical pyrometer without correction for the emissivity of the wire, and cooling to nearly room temperature, according to the cycle used in making the life tests. Since the emissivity of the wire is less than that of a blacl; body, the real temperature of the wire must necessarily have been more than 2300 F., in order to give radiation which afiected the optical pyrometer in the same way as black body radiation corresponding to a temperature of 2300 F. At the end of -517 hours, or 4136 complete heating and cooling cycles, the wire had increased in resistance by 14 per cent, at which time the test was terminated.

For an evaluation of quality, the useful life is arbitrarily taken to be the interval of time which elapses before the alloy increases in resistance by ten per cent. In the test cited,this life was found to be 475 hours.

Wires of the same size, of the best quality of the.

the present alloy was tested without increasing inresistance by ten per cent.

One eighth inch wires of the-same alloy tested at 2600 F., black body temperature, took from 35 to 50 hours to increase in resistance by ten per cent, and lasted over 100 hours in this test. The above-mentioned commercial alloys now available fuse well below this temperature.

In numerous applications the heating elements r previously available are used at temperatures so close to their upper limit of safe operation that upon any overheating'due to failure or control equipment to function properly, they burn out. The present alloy offers an important advantage over these alloys in the temperature range which can be attained by them, in that it possessesa much greater life and can be overheated, without damage, to an extent which would be fatal to resistors made of the other alloys. This alloy possesses an even more important advantage in that it extends the use of metallic resistor furnaces to higher temperature ranges than could.

be reached by resistance elements made of previously known alloys.

An alloy having the above-described composition has a specific resistance of 980 to 1000 ohms per circular mil foot at 70 F., and a temperature coeflicient of resistance of 4.5 per cent between 70 and 2600 F., black body temperature.

We claim:

1. An alloy which comprises approximately 37 per cent chromium and approximately '7 per cent aluminum, the balance being substantially all non.

2. A heating element consisting of anattenuated body of an alloy which comprises approximately 37 per cent chromium and approximately 7 per cent aluminum, the balance being substantially all iron.

3. A heat-resistant alloy which comprises about 3'7 per cent chromium, about 7 per cent aluminum, and about 0.5 per cent titanium, the balance being substantially all iron.

4. A heat-resistant alloy which comprisesabout 37 per cent chromium, about 7 per cent aluminum, and about 0.6 per cent silicon, the balance being substantially all iron.

5. An alloy which comprises chromium in excess of 35 per cent and aluminum'in excess of 5 per cent, the balance of the alloy being greaterthan the combined percentages of chromium and aluminum and being composed of substantially all iron.

6. A heat-resistant alloy which comprises about 37 per cent chromium, about 7 per cent aluminum, and a small amount of a grain refining element, the balance of the alloy being substantially all iron.

7. An alloy consisting substantially of iron, chromium, and aluminum, which comprises chromium in excess of 35 per cent and aluminum in excess of 5 per cent, the balance constituting the major portion of the alloy and being substantially all iron, said alloy having a melting point which is higher than the melting point of pure iron.

SAMUEL L. 11pm. ROBERT s. ARCHER: 

